Fitted exchange rate of the second nucleotide after transfection with differently modified oligonucleotides. Percentage of clones with both exchanges after different transfection experiments with oligonucleotides carrying two mismatches to hprt. The X-axis indicates the distance of the second mismatch (independent of its position in upstream or downstream direction) from the first exchange position. ● first mismatch at position 151, modified by TA-clamps. ◀ : first mismatch at position 153, modified by TA-clamps. ■ : first mismatch at position 151, modified by PTO. The black line shows the linear fit of the data points from the experiments with the three different kinds of oligonucleotides.